Soon enough. Probably early access in september for IP, and later for the general public. Considering that 1903 and 1909 share the same servicing updates, it probably is just one update in difference (kb4517245 1909 Enablement Package).
A CU will be delivered via Windows update but will microsoft also release a new iso going forward? New Windows reinstall are said to be done via the cloud.
If I want to stay on the main channel (not Insider) when 1909 finally goes public, should I install 18363.356 or 18362.10019 ?
Install 1903 (with latest build/update/CU) and then apply KB4517245 Enablement Package if you wanna upgrade to the 1909 without joining the IP ring